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This module provides access to the select() and poll() functions
available in most operating systems, devpoll() available on
Solaris and derivatives, epoll() available on Linux 2.5+ and
kqueue() available on most BSD.
Note that on Windows, it only works for sockets; on other operating systems,
it also works for other file types (in particular, on Unix, it works on pipes).
It cannot be used on regular files to determine whether a file has grown since
it was last read.

Note

The selectors module allows high-level and efficient I/O
multiplexing, built upon the select module primitives. Users are
encouraged to use the selectors module instead, unless they want
precise control over the OS-level primitives used.

devpoll() objects are linked to the number of file
descriptors allowed at the time of instantiation. If your program
reduces this value, devpoll() will fail. If your program
increases this value, devpoll() may return an
incomplete list of active file descriptors.

(Only supported on Linux 2.5.44 and newer.) Return an edge polling object,
which can be used as Edge or Level Triggered interface for I/O
events.

sizehint informs epoll about the expected number of events to be
registered. It must be positive, or -1 to use the default. It is only
used on older systems where epoll_create1() is not available;
otherwise it has no effect (though its value is still checked).

flags is deprecated and completely ignored. However, when supplied, its
value must be 0 or select.EPOLL_CLOEXEC, otherwise OSError is
raised.

(Not supported by all operating systems.) Returns a polling object, which
supports registering and unregistering file descriptors, and then polling them
for I/O events; see section Polling Objects below for the methods supported
by polling objects.

This is a straightforward interface to the Unix select() system call.
The first three arguments are sequences of ‘waitable objects’: either
integers representing file descriptors or objects with a parameterless method
named fileno() returning such an integer:

rlist: wait until ready for reading

wlist: wait until ready for writing

xlist: wait for an “exceptional condition” (see the manual page for what
your system considers such a condition)

Empty sequences are allowed, but acceptance of three empty sequences is
platform-dependent. (It is known to work on Unix but not on Windows.) The
optional timeout argument specifies a time-out as a floating point number
in seconds. When the timeout argument is omitted the function blocks until
at least one file descriptor is ready. A time-out value of zero specifies a
poll and never blocks.

The return value is a triple of lists of objects that are ready: subsets of the
first three arguments. When the time-out is reached without a file descriptor
becoming ready, three empty lists are returned.

Among the acceptable object types in the sequences are Python file
objects (e.g. sys.stdin, or objects returned by
open() or os.popen()), socket objects returned by
socket.socket(). You may also define a wrapper class yourself,
as long as it has an appropriate fileno() method (that
really returns a file descriptor, not just a random integer).

Note

File objects on Windows are not acceptable, but sockets are. On Windows,
the underlying select() function is provided by the WinSock
library, and does not handle file descriptors that don’t originate from
WinSock.

Changed in version 3.5: The function is now retried with a recomputed timeout when interrupted by
a signal, except if the signal handler raises an exception (see
PEP 475 for the rationale), instead of raising
InterruptedError.

The minimum number of bytes which can be written without blocking to a pipe
when the pipe has been reported as ready for writing by select(),
poll() or another interface in this module. This doesn’t apply
to other kind of file-like objects such as sockets.

This value is guaranteed by POSIX to be at least 512. Availability: Unix.

Register a file descriptor with the polling object. Future calls to the
poll() method will then check whether the file descriptor has any
pending I/O events. fd can be either an integer, or an object with a
fileno() method that returns an integer. File objects
implement fileno(), so they can also be used as the argument.

eventmask is an optional bitmask describing the type of events you want to
check for. The constants are the same that with poll()
object. The default value is a combination of the constants POLLIN,
POLLPRI, and POLLOUT.

Warning

Registering a file descriptor that’s already registered is not an
error, but the result is undefined. The appropriate action is to
unregister or modify it first. This is an important difference
compared with poll().

Polls the set of registered file descriptors, and returns a possibly-empty list
containing (fd,event) 2-tuples for the descriptors that have events or
errors to report. fd is the file descriptor, and event is a bitmask with
bits set for the reported events for that descriptor — POLLIN for
waiting input, POLLOUT to indicate that the descriptor can be written
to, and so forth. An empty list indicates that the call timed out and no file
descriptors had any events to report. If timeout is given, it specifies the
length of time in milliseconds which the system will wait for events before
returning. If timeout is omitted, -1, or None, the call will
block until there is an event for this poll object.

Changed in version 3.5: The function is now retried with a recomputed timeout when interrupted by
a signal, except if the signal handler raises an exception (see
PEP 475 for the rationale), instead of raising
InterruptedError.

Changed in version 3.5: The function is now retried with a recomputed timeout when interrupted by
a signal, except if the signal handler raises an exception (see
PEP 475 for the rationale), instead of raising
InterruptedError.

The poll() system call, supported on most Unix systems, provides better
scalability for network servers that service many, many clients at the same
time. poll() scales better because the system call only requires listing
the file descriptors of interest, while select() builds a bitmap, turns
on bits for the fds of interest, and then afterward the whole bitmap has to be
linearly scanned again. select() is O(highest file descriptor), while
poll() is O(number of file descriptors).

Register a file descriptor with the polling object. Future calls to the
poll() method will then check whether the file descriptor has any
pending I/O events. fd can be either an integer, or an object with a
fileno() method that returns an integer. File objects
implement fileno(), so they can also be used as the argument.

eventmask is an optional bitmask describing the type of events you want to
check for, and can be a combination of the constants POLLIN,
POLLPRI, and POLLOUT, described in the table below. If not
specified, the default value used will check for all 3 types of events.

Modifies an already registered fd. This has the same effect as
register(fd,eventmask). Attempting to modify a file descriptor
that was never registered causes an OSError exception with errno
ENOENT to be raised.

Polls the set of registered file descriptors, and returns a possibly-empty list
containing (fd,event) 2-tuples for the descriptors that have events or
errors to report. fd is the file descriptor, and event is a bitmask with
bits set for the reported events for that descriptor — POLLIN for
waiting input, POLLOUT to indicate that the descriptor can be written
to, and so forth. An empty list indicates that the call timed out and no file
descriptors had any events to report. If timeout is given, it specifies the
length of time in milliseconds which the system will wait for events before
returning. If timeout is omitted, negative, or None, the call will
block until there is an event for this poll object.

Changed in version 3.5: The function is now retried with a recomputed timeout when interrupted by
a signal, except if the signal handler raises an exception (see
PEP 475 for the rationale), instead of raising
InterruptedError.

Changed in version 3.5: The function is now retried with a recomputed timeout when interrupted by
a signal, except if the signal handler raises an exception (see
PEP 475 for the rationale), instead of raising
InterruptedError.

Value used to identify the event. The interpretation depends on the filter
but it’s usually the file descriptor. In the constructor ident can either
be an int or an object with a fileno() method. kevent
stores the integer internally.